1. Field of the Invention
The invention pertains to a process for the ultrasonic testing of heat-sealed joints between plastic packages (especially trays for ready-to-eat meals) and cover films. The invention also pertains to equipment for performing this process.
2. Description of the Related Art
Dishlike plastic packages in various shapes for packaging foods are well known. After they have been filled, they are sealed with a cover film, which is not removed until immediately before the food is to be eaten. The stability of foods packaged in this way depends, among other factors, on the tightness of the heat-sealed joint between the cover film and the dishlike package. When the film is applied to the edge of the package, it frequently happens that wrinkles develop, plastic particles become heat-sealed in the joint bulges form, or food particles are trapped between the cover film and the package. If no bond forms between the plastic package and the cover film in such places, the package is not sealed, and then oxygen and contaminants can reach the food through these defects. Furthermore, the food may be able to run out etc. Therefore, to improve the probability that the packaged food will remain safe in accordance with applicable food laws until the expiration date that is printed on the package, it is necessary to make sure that the heat-sealed joint is completely closed all around the package, i.e., that there are no defects in the heat-sealed joint.
Accordingly, the goal of the invention was to develop a process and equipment for testing heat-sealed joints of the type described above. A further goal of the invention was to be able to perform this process with both a high degree of accuracy and reproducibility and with as much speed as possible.